1. Field of the Invention
The present invention relates to a polyimide porous film with a uniform pore diameter and a uniform interpore distance.
2. Description of the Related Art
As a process for the production of porous polyimide films, T. Takeichi et al. describe in High Performance Polymers, Vol. 11, p. 1 (1999) a process for obtaining a polyimide porous film by synthesizing a polyurethane-imide and then heat treating the urethane segment at 300-400xc2x0 C. for decomposition.
In Polymer 36, 1325, 1995 and Polymer 37, 5229, 1996 there is disclosed a process for obtaining porous polyimides by incorporating a readily decomposable xcex1-methylstyrene segment into a polyimide block copolymer or graft copolymer and heat treating the non-polyimide segment for its decomposition.
The present inventors have already invented a porous film possessing throughholes in the film cross-section and having no dense layer on the surface, as disclosed in Japanese Unexamined Patent Publication No. 11-310658. The production process for the porous film accomplishes deposition of a polyimide precursor porous film with throughholes by casting a polyimide precursor solution on a substrate and contacting it with a coagulating solvent via a solvent substitution rate-controlling material.
Japanese Unexamined Patent Publication No. 2001-145826, also by the present inventors, discloses a process for production of a polyimide porous film characterized by employing, as the dope, a mixed solution of a good solvent and a poor solvent for the polyimide precursor.
Polyimide porous films are desired which can be produced cheaply and in a simpler manner without using solvent substitution rate-controlling materials.
When the polyimide precursor dope is cast onto the substrate in the process mentioned above, a difference exists in the conditions of one side of the cast liquid film which is solid, on the substrate side contacting with the substrate, and the other side which is in direct contact with the coagulating solution on the air side opposite it, such that when a porous film is formed by immersion in a coagulating solution, the pore diameters and pore positions are not the same on both sides. This often causes inconveniences in terms of the liquid permeability and gas permeability when the film is used as a battery separator or micro-filter.
Consequently, a process for production of a polyimide porous film with a uniform quality has been ardently desired.
It is an object of the present invention to provide a polyimide porous film with approximately uniform pore sizes and interpore distances.
Specifically, the invention relates to a polyimide porous film obtained by drying and imidizing a polyimide precursor porous film which is substantially homogeneous on both sides, the polyimide porous film having pores on both sides wherein the pores all satisfy the following conditions 1) to 4):
1) The difference in the mean pore size of both sides is less than 200% based on the smaller average value of the mean pore size.
2) The coefficient of variation for the pore size on each side is smaller than 70%.
3) The coefficient of variation for the pore centroid distance on each side is smaller than 50%.
4) The mean pore size on each side is 0.05-5 xcexcm.